Diagnosis: Circular to oval single entrance leading to a cylindrical tunnel that runs closely parallel below the substrate surface and is connected to the latter by a thin, intermittent vane that appears on the surface as a dotted to dashed line of minute openings. The diameter of the main tunnel gradually decreases from proximal to distal, and the keyhole-shaped cross section (where the vane is present) becomes more slender, before ending in a lance-shaped termination. Where the tunnel hits itself or an obstacle, a blind termination is developed and a new branch emerges under an acute angle, circumnavigating the obstacle by delving deeper into the substrate for a short distance.